Subtalar Arthroesis implants, also known as sinus tarsi implants, help treat the hyperpronated foot by stabilizing the subtalar joint. The implant may be designed to block forward, downward, and/or medial displacement of the talus, thereby allowing normal subtalar joint motion while limiting excessive pronation.
Subtalar implants, however, often “back out” or “migrate” from their original implant locations. To mitigate migration, the thread profile of subtalar implants may be aggressive or “deep” to increase the “negative space” of the thread and provide for deeper coupling with tissue. A negative thread space includes, for example, the troughs between the thread crests. In other words, the negative thread space includes the space bordered by the main bodies of two adjacent threads, the core body of the device, and a line connecting the crests of the two adjacent threads. The thread crests are the lateral tips or “crests” of the threads.
However, to create the thread profile for the deep negative space a lathe typically removes significant amounts of material from the thread crest. For example, with conventional thread forms the thread tapers from a wider base to a thinner thread crest. Consequently, machining a larger or deeper negative space requires the removal of additional thread crest area proportionally. As the amount of thread crest surface area decreases the inherent load upon the implant is more focused. This focused load results in higher stress levels for the tissue/implant interface (e.g., pressure points), which may lead to patient discomfort and implant removal.
Pressure points are not caused only by low surface area thread crests. Pressure points may also arise from the general shape of the proximal portion of subtalar implants. For example, conical shaped implants may flare out laterally as the proximal end of the implant is approached. However, the conical shape (as well as cylindrical portions of implants) often terminates proximally in a dramatic non-rounded fashion, thereby leaving a sharp or low-surface area edge and a corresponding pressure point that may possibly be painful.